Setting the time span between a natural or stimulated atrial action and the stimulation pulse for stimulating a ventricular action, the so-called AV delay, is of considerable significance for the hemodynamic optimization of pacemaker function and in connection with preventing pacemaker-induced tachycardia; see M. Schaldach, Electrotherapy of the Heart--Technical Aspects in Cardiac Pacing, Springer Verlag, Berlin, Heidelberg, 1992, pp. 58-60.
Hence, in recent years, pacemakers have been developed in which the AV delay, which was formerly fixed as a function of parameters that reflect the present state of a particular patient in a particular manner, can be changed. The reference basis is the current heart or stimulation rate, with the AV delay decreasing as the stimulation rate increases--see FIG. 30 on page 60 of the above-cited document.
EP 0 450 387 A2 describes an arrangement for automatic setting of the AV delay in a dual-chamber pacemaker, taking into consideration (patient-specific) intra-atrial delay times.
EP 0 494 487 A2 describes an AV-sequential, dual-chamber pacemaker having automatic AV delay programming, in which an objective is to consider pacemaker-stipulated, interatrial and interventricular transmission times. In a test phase prior to the actual pacing, the natural heart rate and a natural AV delay--as a temporal difference between either the natural P wave or an atrial pacemaker "spike" and the natural R wave or a ventricular pacemaker "spike" (with all signals being detected via the pacemaker electrodes)--are determined.
Because this arrangement cannot provide normal stimulation during the test phase, a value of the AV delay obtained from several (for example, four) natural cardiac actions constitutes the basis of a subsequent, long-term stimulation phase (for example, 100 pulses). A truly near-time determination and use of a current AV delay for controlling the pacemaker is therefore not possible, so the pacemaker cannot operate hemodynamically optimally with respect to sudden changes in state, such as the onset or cessation of stresses in the patient.
This disadvantage, of course, also affects the known methods of determining the AV transmission time from a surface EKG, echo-cardiographic methods and the like, because these tests cannot be performed on the patient on a daily, permanent basis.